Flame spraying involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional flame spray gun is used for the purpose of both heating and propelling the particles. In one type of flame spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., below 100 mesh U.S. standard screen size to about 5 microns.
A flame spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas in generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type flame spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together finely divided material, or by bonding together finely divided material by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the material to be sprayed in finely divided form.
Flame sprayed ceramic coatings containing refractories such as zirconium oxide are often used for thermal barrier protection of metal components, such as in gas turbine engines. The zirconium oxide may contain some hafnium oxide and incidental impurities. It typically is stabilized with calcium oxide or yttrium oxide or may be in the form of magnesium zirconate. However, ceramic coatings including these refractories generally are susceptible to cracking and spalling under severe thermal cycling or thermal shock conditions. One reason for this cracking and spalling is that the coefficient of thermal expansion of a ceramic coating is low compared to the metal substrate, resulting in high stresses due to the expansion mismatch.
A particularly severe condition is presented to ceramic coatings on the top of pistons ("piston domes") improving efficiency of internal combustion engines. Such application is disclosed in U.S. Pat. No. 2,978,360 for flame sprayed ceramic coatings categorized as combustion catalysts. However, to date a fully satisfactory coating for this type of environment has not been achieved.
In U.S. Pat. No. 3,625,717 certain flame spray ceramic compositions are taught which have capability under impact and debilitating environments calling for extremely good wear resistance. These prior art ceramic compositions specifically comprise a matrix of aluminum oxide, chromium oxide or stabilized zirconium oxide with an additional phase formed from the matrix oxide with simultaneous additives of iron oxide and titanium oxide.
U.S. Pat. No. 3,645,894 describes plasma spraying spherical agglomerate particles formed by spray drying certain two or three-component metallic oxide powders. The three component particles consist of a first metallic oxide powder characterized by oxygen-ion conductivity upon stabilization, a stabilizing metallic oxide powder and a metallic oxide powder selected from a group consisting of nine oxides. The first constituent may be zirconia or thoria and the stabilizing oxide may be calcium oxide, yttrium oxide, ytterbium oxide or a mixture of rare earth oxides. The group of nine oxides broadly includes titanium oxide, but of a list of 18 examples having more than two components, titanium oxide is entered only once, without elaboration, and is present in combination with a larger portion of iron oxide. The patent does not mention properties such as thermal shock resistance.
U.S. Pat. No. 3,607,343 teaches flame spraying a flame spray powder, the individual particles of which are clad with a fluxing ceramic bonded to the surface thereof. These flame spray powder particles, onto which fluxing ceramic is bonded, are of conventional size and may be any conventional or known flame spray material including metals and ceramic of which zirconium oxide is one example. Fluxing ceramics are certain ceramics having the property of wetting or dissolving other oxide ceramics. Titanium oxide is one example of a number of such fluxing ceramics.
Another known composition is a flame spray powder of zirconium oxide containing calcium oxide and titanium oxide. Coatings of this composition have favorable properties including some thermal shock resistance but the coatings have failed under conditions of extensive use.
Zirconium oxide powder stabilized with yttrium oxide has been mixed with titanium oxide powder and deposited with a plasma gun to produce coatings having high thermal emittance for an x-ray tube, as disclosed in U.S. Pat. No. 4,132,916. A mixture is necessary for this purpose, as distinct from a composite powder, in order that the sprayed titanium oxide retain its identity to contribute a black color to the coating.
Flame sprayed ceramic coatings usually are not fully dense, having some porosity typically up to about 20% depending on composition, powder size distribution, flame spray method and parameters. The high porosity coatings are capable of having a higher degree of resistance to thermal stress than the denser coatings. However, a more porous coating will have lower resistance to erosion and other wear conditions that exist in the environments where such coatings are used. Many high temperature applications also require resistance to erosion by particles and debris.
In view of the foregoing, it is a primary object of the present invention to provide a flame spray material for producing a ceramic coating characterized by a high degree of thermal shock and erosion resistance.
It is a further object of this invention to provide a flame sprayed ceramic coating suitable for use under thermal stress conditions.
It is another object of this invention to provide a flame spray material for producing a ceramic coating that has both high density and a high degree of thermal shock resistance.